Various structures have been attempted in automatic transmissions in order for the power of an engine to be changed to be suitable for a traveling load. However, the automatic transmissions typically have a powertrain structure that includes a gear train having planetary gears, and a hydraulic control unit for operating the gear train.
Such a gear train forms a specific shift stage by combining operations of friction elements, such as clutches or brakes, actuated by the hydraulic control unit. Accordingly, when the hydraulic control unit is actuated according to the traveling of a vehicle and the operation of a driver, a desired gear shift stage is carried out.
The more the automatic transmission has gear shift stages, the better a gear ratio can be properly designed and power output from the engine can be transferred to drive wheels at high efficiency. As a result, it is possible to improve fuel efficiency, acceleration performance, and shift quality.
Thus, it is necessary to develop a powertrain for an automatic transmission having implementable more gear shift stages.
In addition, the durability and power transfer efficiency of the powertrain, the volume and weigh of the transmission, and the like are varied according to the combination of the planetary gear sets and the friction elements, together with the number of gear shift stages which may be implemented in the powertrain of the automatic transmission. Therefore, a research for a compact powertrain having high strength and less power loss is ongoing.
These automatic transmissions are disclosed in Korean Patent Nos. 10-0986542 and 10-0582486.
As a gear train used for a conventional eight-speed automatic transmission adopts at least four sets of planetary gears, the volume, length, and weight of the automatic transmission are increased, thereby causing an increase in weight of a vehicle and poor fuel efficiency.